https://en.wikipedia.org/wiki/Unitary_matrix
Properties. For any unitary matrix U of finite size, the following hold: . Given two complex vectors x and y, multiplication by U preserves their inner product; that is, Ux, Uy = x, y .; U is normal (=).; U is diagonalizable; that is, U is unitarily similar to a diagonal matrix, as a consequence of the spectral theorem.Thus, U has a decomposition of the form =, where V is unitary, and D is ...